Markey, the dean of the state's congressional delegation, also was endorsed by Victoria Kennedy, the widow of the late U.S. Sen. Edward Kennedy, and by U.S. Sen. Michael Bennet, of Colorado, chair of the Democratic Senatorial Campaign Committee.

The apparent closing of ranks behind Markey came one day after the Malden Democrat, who has served in the House since 1976, announced his intention to run for Kerry's Senate seat if it becomes vacant. Kerry was nominated by President Barack Obama to replace Hillary Clinton as secretary of state and is seen as likely to be confirmed by his Senate colleagues.

Also Friday, U.S. Rep. Niki Tsongas, the widow of former U.S. Sen. Paul Tsongas, announced that she would not run for the Senate. But another potential candidate, U.S. Rep. Michael Capuano, a Somerville Democrat, said party leaders would not dictate his decision on whether to run.

Kerry, in a statement, stopped short of formally endorsing Markey but was effusive in his praise for the congressman and said he supported his decision to run.

"Ed's one of the most experienced and capable legislators in the entire Congress, and it would be an almost unprecedented occasion for such an accomplished legislator to join the Senate able to hit the ground running on every issue of importance to Massachusetts," Kerry said.

Victoria Kennedy, who has been mentioned as a possible interim replacement for Kerry until a special election is held, said she believed Markey was the "right person" to succeed Kerry.

"He's respected nationally for his energetic leadership on issues relating to the environment, energy and technology," she said. "He has the experience and expertise to address the pressing economic issues facing our nation."

Bennet, in a statement distributed by the DSCC, also lauded Markey as a leader on major issues.

By moving quickly to embrace Markey, party leaders may be trying to steer the process away from an expensive or divisive Democratic primary involving multiple candidates and clear the field for him to focus on a possible race against Republican U.S. Sen. Scott Brown in the special election.

Brown, who won a special election in 2010 following Edward Kennedy's death but was defeated this year by Democrat Elizabeth Warren in his re-election bid, has not said whether he would run for Kerry's seat.

Tsongas, a Lowell Democrat, didn't refer to Markey in her brief statement on Friday but made clear that she would not join the race, saying she thinks she can best serve her constituents by remaining in the U.S. House.

Other potential Democratic candidates for Kerry's seat include U.S. Rep. Stephen Lynch, of Boston, and state Sen. Benjamin Downing, from Pittsfield, who've said they're considering runs.

Two potential high-profile candidates, Edward Kennedy Jr., a son of the late senator, and actor-director Ben Affleck, took their names out of consideration this week.

If Kerry is confirmed, Democratic Gov. Deval Patrick would appoint an interim senator to serve until the special election is held within about four or five months. Patrick said Friday that Markey would be "a wonderful partner in the Senate" but stopped short of endorsing him, saying he's focused on appointing an interim senator.